The Blockstream “Green Wallet” by Green Address Ltd. is a non-custodial wallet. This means, that you are the true owner of your bitcoin. The Green Wallet gives you access to your Mnemonic seed, which is a 24 word phrase that you need to write down on a sheet of paper and keep good care of it.
Do NOT make a screenshot, do NOT store it on your computer or your phone.
Other wallets for instance the Coinbase wallet, Luno app, Blockchain wallet and many more do not give you access to your recovery data aka mnemonic seed phrase aka backup (many names for the same thing). Which means, that they act like a bank and if their service gets taken down or is hacked your coins are gone too.
Use the Blockstream Green website to ensure you are downloading the real app and not a fake one https://blockstream.com/green/
You can use the Green Wallet on your Android phone, iPhone or on a desktop computer.
Use a wallet on your phone for small amounts only, like your purse. For bigger amounts use a hardware wallet like the BitBox02 https://anita.link/bitbox02